This invention relates in general to photosensitive detectors and in particular to a detector for sensing and analyzing light pulses having extremely short durations and rise times.
Some technologies utilize light pulses of extremely short duration and in order to progress in these technologies one must be capable of analyzing these pulses to ascertain their intensity, duration, build-up and decay characteristics, and the like. Detection and analysis of high speed optical events is most important in laser technology, particularly in the field of laser communications and laser fusion. In these fields, laser pulses having rise times less than 50 picoseconds (a picosecond is one trillionth or 10.sup.-12 of a second) are encountered. Of course, if a photodetector has a response time substantially in excess of the rise time for the light pulse, it is incapable of resolving the light pulse with any measure of precision and a meaningful analysis is not possible.
Texas Instruments markets a packaged photodetector diode known as the TIXL-55 photodiode, and this diode is actually an assembly containing not only a silicon avalanche photodiode, but also a so-called package in which the diode is housed and connections to it are made. This coaxial "pill" package consists essentially of a tubular housing, a lensed window at the end of the housing, and terminals on the housing. In order to use the TIXL-55 diode for light detection, it must be installed in a fixture or mount, through which it is connected with a bias supply and also with an oscilloscope. The package itself as well as the mount adversely affect the speed of response, since they cause the oscilloscope to display distorted images of the actual waveform for a light pulse. For example, reflections often develop in the electrical circuitry, particularly at the capacitor across which the bias is applied, and these reflections appear as subsequent after pulses on the oscilloscope screen. Sometimes they tend to distort the waveform, with the distortion usually appearing at the end of the waveform, making it appear longer than the optical event it actually portrays. Consequently, such devices have relatively poor resolution. A good mount specifically tailored for the TIXL-55 diode is disclosed in U.S. Pat. No. 3,842,262 issued to James L. Heitman and Earl F. Starr, Jr., on Oct. 15, 1974. Even that mount is incapable of resolving light pulses having rise times shorter than 120 psec, partially due to limitations of the photodiode.
The Rockwell GaAsSb avalanche photodiode is capable of resolving light pulses having rise times as short as 50 psec, but it operates in a narrow spectral range in the vicinity of 1064 nm. Certain high speed vacuum diodes resolve light pulses of equivalent rise times, but these diodes cannot tolerate sufficient operating current to be useful in observing the high average signal levels of many laser devices, such as mode locked cw lasers.